1. Field of Invention
This invention relates to the conversion of chemical energy to electrical energy. In particular, the present invention relates to an implantable medical device powered by an alkali metal electrochemical cell, such as of lithium coupled with a sandwich cathode. The sandwich cathode design comprises a second cathode active material of a relatively high energy density but of a relatively low rate capability sandwiched between two current collectors with a first cathode active material having a relatively low energy density but of a relatively high rate capability in contact with the opposite sides of the current collectors. The present invention then provides an indicator as to when the cell's discharge capacity is nearing end-of-life (EOL) based on the theoretical capacity and the discharge efficiency of the first and second cathode active materials. This early warning is defined as the elective replacement indicator (ERI) and signals a physician when it is time to replace the medical device. Suitable medical devices include cardiac defibrillators, neurostimulators, pacemakers, and the like.
2. Prior Art
In implantable medical device applications, it is important to be able to accurately predict when the batteries powering the device will reach their end of life. For patient safety, doctors need to have this information several months before the battery reached the end of its service life. This gives them time to schedule the patient for replacement of the implantable device.
Historically, the Li/SVO cell system has been used as the power source for implantable cardiac defibrillator applications requiring high rate pulse capability, i.e., about 1 to 4 about 4 amps. Since Li/SVO cells have a staged discharge voltage profile, a pre-determined background voltage is generally used as the ERI. This pre-determined voltage value varies depending on the cell size, theoretical capacity and the associated device design. Additionally, due to the characteristic voltage delay and Rdc growth that occurs at about the 2.6-volt plateau, a pre-determined Rdc or voltage value under high current pulsing is sometimes used as an ERI indicator. Consequently, the ERI indicator selection is very complicated and dependent on the individual device design of each manufacturer.
The Li/CFx system provides medium rate discharge capability (mA range). This cell is a good power source for devices like implantable neurostimulators and implantable devices that treat cardiac heart failure (CHF). Although the Li/CFx system has very high energy density, its discharge voltage profile is mostly flat (˜2.8V). Near the end of discharge, however, a sharp voltage drop occurs. This unfavorable characteristic voltage profile makes it very difficult to set the ERI accurately for Li/CFx cells. In order to resolve this problem, mixtures of CFx and other cathode materials, for example SVO, are proposed.
U.S. patent application Ser. No. 09/560,060, filed Apr. 27, 2000, titled “Sandwich Cathode Design For Alkali Metal Electrochemical Cell With High Discharge Rate Capability” is assigned to the assignee of the present invention and incorporated herein by reference. This application describes a cathode construction with the configuration of: first cathode active material/current collector/second cathode active material/current collector/first cathode active material. In this design, the first cathode active material has a relatively lower energy density but a relatively higher power capability than that of the second cathode active material. In a design of: first cathode active material/current collector/second cathode active material, the first material always faces the anode. While the concept of using these types of electrodes in electrochemical cells has been disclosed, the utilization of such a system to address the ERI/EOL issue remains unresolved. In the present invention, the utilization of this cathode design concept to address the ERI/EOL issue is discussed.